1. Area of the Art
The present application concerns desalination of ocean water and specifically a new method to get pure water easily.
2. Description of the Prior Art
The world faces serious water problems. The increased warming of the ocean surface brought on by increases in atmospheric carbon dioxide and other “greenhouse gases” is a big problem for island countries. Increasing temperatures cause the oceans to rise as polar ice caps melt. This may drown the islands. Meanwhile, elevated water temperatures damage the coral reefs so that island dwellers also face a shortage of food. At the same time increases of atmospheric temperature promote desertification. Desert dwellers lack water for direct consumption and for agriculture. Desert dwellers also face a food shortage. Both of these problems are environmental. All living things contain water as a major constituent of their bodies. Most biological functions and many biological structures depend on water and are not possible under a dry or dehydrated state. “Dry” means “death” for most living creatures.
Therefore, the problem of water shortage is very important for all humans. By the middle of the twenty second century it is estimated that world population will exceed 10 billion. This rapid increase in the world's population will increase the already intense competition for water, food and energy. These problems are closely related to each other and are not readily separable.
Most water on Earth is ocean water (seawater). Pure water exists in ice (glaciers and snow), rain, rivers, ponds, lakes, and underground aquifers. The agriculture use of water depends on the above-mentioned water sources. Unfortunately, many aquifers contain water rich in minerals that accumulate in the soil following prolonged irrigation. This accumulation causes the so-called “salt injury” to plants. Where the water is high in sodium, the actual structure of the soil is damaged as sodium replaces calcium in clay minerals.
The 97.5% of the planet's water is ocean water, which cannot be used agriculturally without the removal of salt—desalination. There are basically two popular desalination processes: distillation and membrane separation. Distillation is a process in which water molecules evaporate from seawater and are subsequently condensed as pure water. The membrane separation process involves either “electron dialysis” (the ED method) or “Reverse Osmosis” (the RO method).
Distillation yields pure water and the residue from this process is salt. The membrane processes ED and RO trap ions and salts so the residue is pure water. Distillation requires much energy for heating the water to accelerate evaporation whereas and the membrane processes require expensive membranes.
The practical problem of the desalination of ocean water is one of performance at an industrial level since such huge amounts of water are needed for agriculture on a scale that can convert the deserts into green plantations. Photosynthesis is the only significant process by which living organisms capture solar energy and store it by synthesizing glucose from carbon dioxide and water. It is a well-known rule of thumb that the growth of agricultural crops requires a weight of water about one thousand times the weight of the harvested crop.
Standard ocean water contains about 3.4 weight % of salts with a pH of 8. The most prevalent anions are the chloride ion (19,000 mg/l) and the sulfate ion (2,600 mg/l). The common cations are the sodium ion (10,650 mg/l), the magnesium ion (1,300 mg/l), the calcium ion (400 mg/l) and the potassium ion (380 mg./l.) There are also lower levels of the bromide ion, the carbonate ion, the boron ion and the strontium ion. In addition there are traces of iron, silica, and calcium carbonate.
Each of these ions has a diameter of several Angstroms (10−10 m). The hydrated ions are in the same dimensional order of magnitude. The separation efficiency of the membrane desalination processed is owed to the ion trapping ability and the ion permeability of the membranes. The affinity and the pore sizes of the membranes are key to the separation results. The manufacture of the membranes must be carefully done and special polymers are required. For the ED method ion-exchange polymers are favored whereas for the RO method cellulose acetate and similar neutral polymers are preferred. Modification of the polymer matrix, for example by cross-linkage and side-chain modification, has been tried to improve efficiency. Nevertheless, it is still difficult to remove all ions.
On the other hand, the distillation method is a standard laboratory method for separation of a liquid from various contaminants. After the removal of particulate matter, ocean water is boiled and evaporated. The resulting steam is then cooled to condense it into liquid water. The common fractional distillation tower is able to yield a large amount of pure water. The problem is to use energy efficiently. A number of technologies attempt to decrease the total number of calories needed to distill pure water from ocean water.
For producing drinking water it is necessary to remove essentially all ions, because our body already contains a critical balance of most of the ions found in ocean water and can be damaged by additional ions. For use in agriculture it is also necessary to remove essentially all ions. As mentioned above, the accumulation of salts in agricultural lands renders the soil non-arable so that the farmer must give up cultivation of the affected land. In the world there are many regions where this has already occurred. There are a myriad of problems resulting from shortage of water—formation of deserts, salt injury to agriculture, as well as lack of drinking water. These problems necessitate complex systems for the long-distance transport of water as well as “water wars” when water is taken from one region to benefit another.